I'm not sure how to make them all the same size, but I do know how to put a size limit so that they can't get any larger than you specify, if that interests you and no one can help you with your problem. =) Hope you find the answer, though.

Yeah. That code is working only for images from signature. If you want to limit characters, you can, but for all users.

You can also, enable for only groups you want signatures. Administration Panel => Users & Groups => Users => User options => User Settings => Allow Signatures : Groups. Select the only groups. You can make a hideen group, and there put the members wich have signature enabled.